Beam Delivery System Risk Issues

1
Beam Delivery System Risk Issues

• American Linear Collider Physics Meeting
• SLAC
• January 8th, 2004

2
Introduction

• Analyze risks to the LC project completion
• Considered four categories
• Type beam physics engineering production
• Impact impact on luminosity or energy reach
• Time when the problem would be uncovered
• Consequence impact of fixing the problem
• Rankings in each category were then multiplied
  together
• Risk is evaluated against the design parameters
  E Lum.
• Risks is based on present evaluation
• Many risks will change as RD progresses
• Only considered a subset of relevant items
  broad scope
• A total of 40 items for each of US warm and US
  cold are listed

3
Example 1 SLED-II

• SLED-II Demonstration
• Technology State of the art 4
• Effect linear impact on energy 3
• Time RD Stage 1
• Consequence Back to RD 4
• Total 48
• SLED-II Production
• Engineering Feasible but untested 3
• Effect linear impact on energy 3
• Time PED Stage 2
• Consequence Major rework 3
• Total 54
• SLED-II Operations Example (not actually
  included)
• Total 36

4
Example 2 Active Vibration Suppression

• Demonstration Example (not actually included)
• Technology RD prototypes but extrapolation
  remains 3
• Effect impact on luminosity is quadratic or
  steeper 4
• Time RD Stage 1
• Consequence Back to RD 4
• Total 48
• Operations
• Engineering Feasible but untested 3
• Effect impact on luminosity is quadratic or
  steeper 4
• Time Pre-ops Stage 3
• Consequence Major rework 2
• Total 72
• Many items identified in BDS were high risk
  because uncovered late in the project cycle

5
Risk Evaluation

• High risks are attached to issues that are not
  understood or have not been demonstrated
• Risks are high when issues are demonstrated late
  in the project cycle
• One problem all of us understand the warm better
  than cold
• Much of cold design is based on the TESLA TDR but
  this has lots of known errors (and possibly a few
  unknown errors)
• I think we overcompensated in an attempt to be
  unbiased

• E source target damage
• E source operations impact
• Ions and e-cloud in DR
• DR impedance
• Collimation system efficiency
• Single tunnel LC design

• E source yield
• DR dynamic aperture
• DR tolerances
• Emittance growth in LET
• Head-on collision extraction
• IP feedback

6
BDS Risks

• Compiled by Mike Harrison and myself
• Much of the BDS is conventional
• Elements which are more novel include the
  superconducting final focusing magnets, the beam
  collimators, the vibration suppression systems,
  and the fast feedback systems
• Beam dynamics issues which is novel are related
  to the short bunches, the higher energy, and the
  small beam emittances
• Operation of the BDS depends on the input beams
• Emittances are designed to be the same
• One significant difference between warm and cold
  is the incoming beam jitter
• Another difference is the pulse structure

7
Table of LC BDS Parameters
8
Emittance and Jitter Budgets

• LET simulation codes benchmarked against each
  other
• Schulte and Walker, PAC 2003 and PT get similar
  results for the linacs
• 40 growth through the linacs ? round up to 50
• Some BDS tolerances tighter for cold and some
  looser
• Warm BC more complicated but lower DE/E
• Estimate for De/e larger in cold BC than in warm
  but

9
LC Environment
Simulation of beam-beam interactiondebris in NLC
IR (e- from left)
Not quite as clean as people might like!
BPM measurements on PEP-IIIR BPMs during abort
gap
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BDS Risks (1)

• Backgrounds 81
• Beam physics Poor or ambiguous data indicates a
  problem 3
• Effect linear impact on luminosity 3
• Time Pre-Ops Stage 3
• Consequence Major redesign 3
• Why is there a risk?
• We can model and design extensively now, but,
  turn the machine on and !
• This is the experience of most colliding beam
  facilities
• Hard to fully model all parts of the problem
• The LC is probably in better shape because we are
  so concerned
• Calculated beam tails are similar in warm and
  cold designs at 10-6 of the beam calculations
  are incomplete

11
BDS Risks (2)

• Final Magnet Stabilization 72 (warm) 0 (cold)
• Engineering/Design Feasible but untested 3
• Effect Quadratic or steeper impact on luminosity
  4
• Time Pre-Ops Stage 3
• Consequence Minor redesign 2
• Why is there a risk?
• Natural motion should be less than 20 nm based
  on SLD measurements
• Want to stabilize at the 0.5 nm level
• Done in other cases but not in the IR environment
• Important for operation (FONT may provide some
  backup)
• Possible to develop in the lab and build a full
  mock-up during the PED phase (there is some risk
  associated with the lab development)
• However, impossible to fully duplicate actual
  installation

12
Scenario 1 No stabilization, no FONT, quiet
detector. Scenario 2 No stabilization, need
FONT, noisy detector. Scenario 3 Stabilization,
no FONT, noisy detector.
13
BDS Risks (3)

• IP Feedback Implementation 48 (warm) 72
  (cold)
• Engineering/Design RD prototype 2 (warm)
• Engineering/Design Feasible but untested 3
  (cold)
• Effect Quadratic or steeper impact on luminosity
  4
• Time Pre-Ops Stage 3
• Consequence Minor redesign 2
• Note categories need to be interpreted broadly
• Why is there a risk?
• Absolutely essential for luminosity after a few
  seconds
• Unprecedented requirements sub-nm accuracy
• Why is cold harder than warm?
• Higher resolution required for the same DL/L
• More complex system multiple interacting
  feedbacks

14
Beam-Beam Deflection Resolution

• Required resolution is determined by the outgoing
  angles
• Tolerances are 1.5 2x tighter in cold LC

15
Outgoing Distribution

• High disruption makes the outgoing distribution
  highly nonlinear
• May be difficult to determine centroid
• RF bpms may not work
• It appears that close to maximal luminosity is
  attained when the beam-beam deflection centroid
  is minimized

16
Solenoid and Crossing Angle

• Strong solenoid with the crossing angle will
  cause variation of the vertical trajectory with
  the horizontal position and with the energy loss
• These may degrade theeffective resolution
• Outgoing spectrum hasa large fraction of
  beamparticles at less than 50 energy
• Low energy particleswill get large
  deflectionsand may cause backgrounds

17
More Complex Feedback System (1)

• The higher disruption and the larger incoming
  beam jitter of the cold LC requires two linked
  feedback systems
• TDR design has angle feedback 850 meters
  upstream of IP
• Both angle and position setting change from
  pulse-to-pulse
• Beam trajectory changes from pulse-to-pulse by
  sigma
• Impact of BDS wakefields has not been considered
• Trajectory changeswill generate varyingbeam
  tails
• TDR design has 5 DN/N ? trajectory changes
  from bunch-to-bunch

Figure 7.18 from TRC
18
More Complex Feedback System (2)

• Changing IP angle through BDS will confuse BDS
  drift feedbacks
• Drifts feedbacks are required to stabilize the
  trajectory at the BDS sextupoles at the sub-um
  level
• 1-sigma angle change corresponds to 100 um
  trajectory change
• Cold LC may need intra-train luminosity feedback
  as well as position and angle feedback
• Require fast luminosity monitor that will not be
  impacted by changes in backgrounds
• Beamstrahlung spectrum, energy loss, and
  deflections are very sensitive to collision
  parameters and tails
• Higher bandwidth not a fundamental limitation but
  complicates implementation
• 3 MHz feedback requires significant faster
  processing ? much faster BPMs and kickers

19
Simulation Results

• Early TDR simulations were incomplete
• Glen White has performed full simulations of
  TESLA system - still work in progress
• Results published at PAC03 by Schulte, Walker,
  White showed an average luminosity of 2.2e34
  result below presented at SLAC
• Each case depends on trajectory jitter see
  Figure 7.18 from TRC
• No wakefieldsand no correlationsbetween
  backgroundsand trajectory

Nominal L 3.4e34
20
Summary

• Many other risk issues identified in BDS
• Collective effects
• Magnet jitter in BDS
• Heating of SC IR magnets
• Collimator performance and MPS limitations
• Aberration tuning procedures
• Crab cavity
• The upper 3.5 items are also issues that can only
  really be determined late in the project cycle
• Risks in the BDS are high because, although
  unlikely, there is significant luminosity impact
  and little time for remediation
• Given present knowledge, the risks in warm and
  cold BDS are very similar